1. Field of the Invention
The present invention relates to an aluminum nitride (AlN) sintered body, a method for manufacturing the same, and a ceramic circuit board including the sintered body as an insulating layer.
2. Description of the Related Art
An aluminum nitride (AlN) sintered body is used as a radiating circuit board for mounting semiconductors since its thermal conductivity is higher than that of alumina or the like and its thermal expansion coefficient is close to that of silicon (Si). The AlN sintered body also has excellent characteristics, such as a high strength at high temperatures and a low reactivity with molten metals. Therefore, the sintered body is beginning to be applied to various other fields. Recently, as researches for increasing the thermal conductivity of the AlN sintered body have advanced, 200-W/m.K class sintered bodies and packages have been able to be obtained in many places.
To achieve a high thermal conductivity in the AlN sintered body, it is considered necessary to decrease the heterophase amount in grain boundaries for the purpose of decreasing phonon scattering in the grain boundaries, thereby increasing the grain size. For this purpose, it has been assumed necessary to sinter an AlN powder at a high temperature around 1,800.degree. C. for a long time period to thereby grow grains of the sintered body to have a grain size of several .mu.m to between 10 and 20 .mu.m. However, the growth of crystal grains accompanies a problem of a decrease in mechanical strength. This decrease in the mechanical strength of the AlN sintered body is a serious problem in applying the sintered body to a circuit board or a package for mounting semiconductor chips.
A high thermal conductivity of the AlN sintered body is achieved by adding a sintering agent, such as an alkaline earth metal compound or a rare earth element compound, which traps impurity oxygen of AlN crystal grains, and sintering the resultant material at a high temperature. However, the AlN sintered body whose thermal conductivity is raised by such a method is poor in mechanical characteristics. A circuit board for mounting semiconductor chips must have an enough strength to withstand a thermal stress or other mechanical stresses, as well as a high thermal conductivity. For this reason, the advent of an AlN sintered body having both a high thermal conductivity and a high strength has been desired strongly. Since, however, a high thermal conductivity and a high strength are contradictory characteristics from a conventional commonsense point of view, almost no AlN sintered bodies having both the characteristics have been reported.
The mechanism of sintering and the relationship between the thermal conductivity and the strength of the AlN sintered body will be described in detail below. Generally, an aluminum oxide impurity unavoidably contained in an AlN powder reacts with a sintering agent, such as a rare earth element compound, to produce aluminates during sintering. These aluminates produce a liquid phase during the sintering, remaining in the edges and the triple points of AlN grain boundaries. In other words, the aluminum oxide impurity is trapped in a grain boundary phase, and the purity of the AlN crystal itself increases. As an example, the use of Y.sub.2 O.sub.3 as a sintering agent produces a Y-Al-O-based composite oxide, such as 3Y.sub.2 O.sub.3.5Al.sub.2 O.sub.3 or Y.sub.2 O.sub.3.Al.sub.2 O.sub.3, and the use of CaO as a sintering agent forms a Ca--Al--O-based composite oxide, such as CaO.Al.sub.2 O.sub.3 or 2CaO.Al.sub.2 O.sub.3. It is assumed that liquid phase sintering of AlN progresses to increase its density because of the presence of such a composite oxide grain boundary phase. As can be predicted from the eutectic points of the above individual composite oxides, the sintering temperature for obtaining the AlN sintered body is approximately 1,800.degree. C. when the rare earth-based sintering agent is used. Although the sintering temperature when the alkaline earth metal-based sintering agent is used is slightly lower than that when the rare earth-based sintering agent is used, there is no large difference between the two temperatures.
When a long-time sintering in a reducing atmosphere at a temperature of 1,800.degree. C. or more is combined with the addition of the above sintering agent, the thermal conductivity further increases to make the manufacture of an AlN sintered body of about 270 W/m.K possible. This is so because a grain boundary phase composition changes during the high-temperature, long-time sintering, reducing solid solution oxygen contained in AlN grains. Since, however, the high-temperature, long-time sintering also causes growth of crystal grains simultaneously, the mechanical strength decreases from 40 to 50 kg/mm.sup.2 to 30 kg/mm.sup.2. As described above, a high thermal conductivity and a high strength are contradictory characteristics in the AlN sintered body, so it has been very difficult to improve both the characteristics at the same time.
As for applications of the AlN sintered body, on the other hand, techniques to form the sintered body into various parts and to mass-produce the sintered body have been studied in recent years. In particular, decreasing the manufacturing cost is an urgent necessity to widen the range of applications of the AlN sintered body, and so a demand has arisen for sintering at a low temperature of 1,700.degree. C. or less. In such low-temperature sintering, however, it has been impossible to obtain an AlN sintered body having both a high thermal conductivity and a high strength.